chore(nix): add turbo to flake setup

[venikx]

Fixes the following:

• running pnpm dev using the nix flake setup resulted in an error related to turbo
• running pnpm build using the nix flake setup resulted in an error related to turbo

Downsides:

• turbo needs to be pinned down the exact version from in the nix flake
• the nix flake lock was from last year, meaning turbo wasn't part of that yet => required nix flake update => nix complained about node 16 being deprecated and insecure => node 18, with pnpm has a newer version of the lockfile, so the whole lockfile got updated.

But, I'm uncertain if you are confident about using node 18, since node 16 was deprecated or not, because the pnpm lock file has a change as well due to this.

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: b656509c17808f3b878774f13dfbb3cdd2eb76e6

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[rasendubi]

ok, I found that CI error is actually cause pnpm version mismatch (lock file used 8.x, but CI still used 7.x)

[rasendubi]

yay! thanks for the PR